The invention relates to diaphragm pumps. More particularly, the invention relates to improvements in diaphragm pumps of the type wherein the marginal portion of a flexible diaphragm is secured to the pump housing and a central portion of the diaphragm is reciprocable by a connecting rod or in another suitable way so as to draw a fluid medium into a variable-capacity pumping chamber during movement in a first direction toward one end position, and to expel the fluid medium from the chamber during movement in the opposite direction to a second end position in which the volume of the pumping chamber is reduced to a minimum.
It is already known to provide the diaphragm of a diaphragm pump with a relatively thick central portion which is coupled to a connecting rod serving to convert rotary movements of a driving member (such as an orbiting crank pin) into reciprocatory movements of the central portion of the diaphragm. The marginal portion of the diaphragm is clamped between two sections (e.g., between a case and a cover) of the pump housing, and one side of the diaphragm defines with the inner side of the cover a variable-capacity pumping chamber which receives a fluid medium by way of one or more inlets while the central portion of the diaphragm is caused to move away from the cover. The fluid medium is expelled from the chamber during the next-following exhaust stroke, i.e., while the central portion of the diaphragm is being moved toward the cover so as to reduce the volume of the pumping chamber. The intermediate portion of the diaphragm is relatively thin and hence readily flexible in order to permit deformation of the diaphragm in the region between the reciprocating central portion and the clamped marginal portion.
It is further known to impart to the inner side of the cover a substantially concave (spherical shape) and to impart to the adjacent front side of the diaphragm a complementary convex shape. This ensures that the front side of the diaphragm can more closely conform to the shape of the inner side of the cover when the diaphragm completes its exhaust stroke. Such diaphragm pumps are often used as vacuum pumps.
A drawback of heretofore known vacuum pumps which employ a diaphragm of the above outlined character is that their fluid evacuating capacity is rather limited. One of the primary reasons for the inability of conventional diaphragm pumps to effect a more satisfactory evacuation of fluids from vessels or the like is that at least a certain part of the relatively thin and readily flexible intermediate portion between the central and marginal portions of the diaphragm often remains spaced apart from the inner side of the cover when the diaphragm completes its exhaust stroke, i.e., the volume of the pumping chamber is not reduced to zero and a certain amount of fluid medium remains in the pumping chamber while the central portion of the diaphragm abuts the inner side of the cover. It has been found that a conventional vacuum pump is incapable of establishing a vacuum in excess of approximately 75 Torr, and the main reason is the presence of dead space between the intermediate portion of the diaphragm and the cover of the pump housing while the central portion of the diaphragm abuts the adjacent concave portion of the inner side of the cover.
German Auslegeschrift No. 22 11 096 of Erich Becker discloses a diaphragm pump wherein the inner side of the cover of the pump housing is provided with a convex portion which is adjacent the outlet and conforms to the shape of adjacent portion of the front side of the diaphragm during the last stage of movement of central portion toward abutment with the concave central portion of the inner side. Thus, the configuration of the inner side of the cover departs from the shape of adjacent portions of the inner side for the express purpose of ensuring that it can be contacted by that portion of the front side of the diaphragm which is last to reach its foremost position (namely a position nearest to the cover of the pump housing). The pump of Becker is capable of considerably reducing the dead space between the diaphragm and the cover (i.e., of considerably reducing the minimum volume of the pumping chamber) when the diaphragm completes its exhaust stroke. Therefore, the pump of Becker has found wide acceptance in many branches of the industry.
A drawback of the pump of Becker is that the central portion of the diaphragm contains a relatively large mass of elastomeric material. Thus, the diaphragm must perform a pronounced fulling action during each of its exhaust strokes; such fulling entails heating and shortens the useful life of the diaphragm. Moreover, pronounced deformation of the diaphragm during the exhaust stroke reduces the ability or capacity of the pump to draw a fluid medium into its pumping chamber. Still further, the making of a cover with a specially designed inner side contributes significantly to the initial cost of the pump.